Claims that the new Routemaster bus is the “most environmentally” friendly of its kind have come under renewed scrutiny today after it emerged that Transport for London has failed to test the vehicle’s environmental impact when operated in diesel-only mode.

Last month Labour Mayoral hopeful Christian Wolmar published a dossier compiled by drivers of the buses detailing how battery failures force the supposedly hybrid bus to operate on its diesel engine for prolonged periods.

It’s now emerged that bosses at TfL have failed to test the level of engine emissions generated by the bus when operating solely on the diesel engine.

Responding to a Freedom of Information request submitted by Liberal Democrats on the London Assembly asking for emissions data in all modes, TfL said: “We only hold hybrid mode emissions data, as the basis of the test is to assess the vehicle in its normal operating mode.”

Stephen Knight AM, the Liberal Democrat Assembly environment spokesperson, said the response called into question TfL and City Hall’s claims about the vehicle’s eco-credentials.

He said: “Time and time again the Mayor has claimed that the new Routemaster is the most environmentally friendly bus of its kind. Such a claim is simply nonsense if the buses are not operating as intended and relying entirely on their dirty diesel engines.

“Londoners have paid a fortune for the new Routemasters, at the very least they deserve to know how polluting they really are.”

Mr Knight has now written to the Mayor urging him to commission new testing to determine the level of emissions the buses produce in all modes.